1. Field of the Invention
The present invention relates to an electronic weighing apparatus. More specifically, the present invention relates to a zero tracking circuit in an electronic weighing apparatus.
2. Description of the Prior Art
A typical prior art electronic weighing appratus comprises a load converter responsive to a load being weighed for generating an electrical analog signal representative of the load, a direct current amplifier for amplifying the output from the load converter, an analog-digital converter for converting the electrical analog signal from the direct current amplifier into an electrical digital signal, and a digital display responsive to the electrical digital signal from the analog-digital converter for displaying the load or weight in a digital manner. The load converter may comprise a resistor wired bridge type load cell, for example.
Such an electronic weighing apparatus of a relatively simple structure is subject to a minor fluctuation of the zero point of the output by virtue of the environmental changes such as of temperature, humidity and the like and the time dependent changes of the characteristics of the components. Such a minor fluctuation brings about a situation where some output is displayed in the digital display although no load has been placed on the load converter.
An improved prior art electronic weighing apparatus where the above described problem as to a minor fluctuation of the zero point has been solved is shown in FIG. 1. Referring to FIG. 1, the prior art electronic weighing apparatus comprises a load converter 1 responsive to a load being weighed for generating an electrical analog signal representative of the load, a direct current amplifier 2 for amplifying the output from the load converter 1, an analog-digital converter 3 of a conversion enable signal controlled type for converting the electrical analog signal from the direct current amplifier into a corresponding electrical digital signal, a periodical enable signal generator 4 for providing periodically a conversion enable signal to the analog-digital converter 3, and a digital display 9 responsive to the electrical digital signal for displaying the value in a digital manner. A zero tracking circuit 5 is provided for eliminating the above discussed problem of a minor fluctuation of the zero point between the analog-digital converter and the digital display 9. The zero tracking circuit 5 comprises a store circuit 6 responsive to a store enable signal for storing an electrical digital signal from the analog-digital converter 3, a digital subtracter 7 for receiving the electrical digital signal from the analog-digital converter 3 at the minuend input and for receiving the electrical digital signal stored in the store circuit 6 at the subtrahend terminal, and a store enable signal generator 8 for providing a store eneble signal to the store circuit 6. The store enable signal generator 8 may be manually operable to provide a store enable signal in response to manual operation.
In operation, prior to the measurement, the store enable signal generator 8 is manually operated for the purpose of zero tracking. A store enable signal is applied from the store enable signal generator 8 to the store circuit 6, whereby the store circuit 6 serves to store the output from the analog-digital converter 3. Since in such a situation no load has been placed on the load converter 1, the electrical analog signal representative of the zero point fluctuated value obtainable from the load converter 1 in such a situation is applied to the direct current amplifier and the zero point fluctuation value, as amplified, is applied to the analog-digital converter 3. The analog-digital converter 3 is responsive to the conversion enable signal from the conversion enable signal generator 4 to cause the analog-digital conversion of the above described zero point fluctuation value, whereby an electrical digital signal representative of the zero point fluctuation value is generated. Thus, it is understood that if and when the store enable signal generator 8 is operated while no load is placed on the load converter 1, an electrical digital signal representative of the above described zero point fluctuation value is stored in the store circuit 6.
Then an article to be weighed is placed on the load converter 1. An electrical analog signal representative of the load is obtained from the load converter 1 and is applied to the direct current amplifier 2. The electrical analog signal, as amplified, is converted into an electrical digital signal by means of the analog-digital converter 3 as a function of the conversion enable signal. The electrical digital signal representative of the load obtainable from the analog-digital converter 3 is applied to the minuend input of the digital subtracter 7. On the other hand, the electrical digital signal representative of the zero point fluctuation value as stored in the store circuit 6 is applied to the subtrahend input of the digital subtracter 7. As a result, the digital subtracter 7 subtracts the electrical digital signal representative of the above described zero point fluctuation value from the electrical digital signal representative of the load after the article being weighed has been placed on the load converter 1, thereby to provide an electrical digital signal corresponding to the difference and as compensated relative to the zero point fluctuation to the digital display 9. Thus, the digital display 9 displays the load of the article being weighed as compensated relative to the zero point fluctuation.
According to the FIG. 1 zero tracking circuit, when the store enable signal generator 8 is manually operated, the zero point fluctuation value is unconditionally stored in the store circuit 6 irrespective of any periodicity of the zero point fluctuation. As a result, an instantaneous fluctuation of so small a period that does not affect the weighing measurement, such as a noise, a pulsing load, a minor vibration and the like, is also treated as a zero point fluctuation and the electrical digital signal from the analog-digital converter 3 corresponding to such an instantaneous fluctuation is also stored in the store circuit 6 and hence the digital subtracter 7 subtracts the electrical digital signal caused by such an instantaneous fluctuation, from the electrical digital signal corresponding to the load after the article being weighed is placed on the load converter 1. However, such an instantaneous fluctuation that does not affect the weighing measurement should be inherently disregarded. Thus, with the FIG. 1 zero tracking circuit, a problem is encountered that such an instantaneous fluctuation that should be disregarded is also treated as a zero point fluctuation, thereby to make compensation, resulting in an error in the weight measurement.